In the related art, a directional coupler described in, for example, Patent Literature 1 (Japanese Unexamined Patent Publication No. 2013-5076), is known. The directional coupler described in Patent Literature 1 includes a first terminal to a fourth terminal, a main line connected between the first terminal and the second terminal, a first sub line that is connected to the third terminal and is electromagnetically coupled to the main line, a second sub line that is connected to the fourth terminal and is electromagnetically coupled to the main line, and a phase converter that is connected between the first sub line and the second sub line and generates a phase shift in a passing signal. In the directional coupler, the main line, the first sub line, and the second sub line are disposed between a pair of ground layers that are connected to the ground.